The present invention relates to a process for the preparation of highly pure aromatic diurethanes and/or polyurethanes by the reaction of aromatic diamines and/or polyamines with unsubstituted carbamates with the release of ammonia.
The preparation of aromatic diurethanes and/or polyurethanes from aromatic diamines and/or polyamines and unsubstituted carbamates is of great interest as aromatic diurethanes and/or polyurethanes can be converted by thermal decomposition into aromatic diisocyanates and/or polyisocyanates which may be used as starting materials for the production of high quality polyurethane resins. Since the unsubstituted carbamates used as reactants for the preparation of the aromatic diurethanes and/or polyurethanes are obtainable from urea and alcohols, they enable aromatic diisocyanates and/or polyisocyanates to be produced without the use of phosgene.
Processes for the preparation of aromatic diurethanes and/or polyurethanes from aromatic diamines and/or polyamines and unsubstituted carbamates are known and are described in numerous patents (DE 2,942,511, DE 2,917,568, DE 2,943,480, EP 18,583). The processes hitherto employed have the disadvantage that the diurethanes and/or polyurethanes are not obtained in pure form. A high purity is necessary for the successful decomposition of urethanes, and hence for the optimum preparation of aromatic diisocyanates and/or polyisocyanates. The products produced by the known processes generally contain starting materials and/or by-products which are inevitably produced, e.g. aminourethanes, urea urethanes, oligoureas and polyureas. These impurities can only be separated with great difficulty from the products of the process, and therefore, increase the cost of preparation.
One exception are the aromatic diurethanes based on methanol obtained from the reaction of aromatic diamines and methyl carbamate. In general, these urethanes have a high melting point, good crystallization properties, and low solubility in organic solvents. Therefore, in contrast to aromatic diurethanes based on other alcohols, they separate as pure crystalline solids from the reaction mixture or can be separated relatively easily by recrystallization from the unwanted impurities which interfere with the decomposition of urethanes. The high melting point and low solubility of the diurethanes based on methanol have, however, a disadvantageous effect on the use of the end products for the preparation of isocyanates. Conversion of the products of the process into isocyanates is carried out by the thermal decomposition of the underlying urethanes. This decomposition is carried out technically at temperatures above 200.degree. C., in a decomposition apparatus into which the urethanes are continuously introduced in the molten or dissolved state (see e.g. U.S. Pat. No. 4,388,246, U.S. Pat. No. 4,081,472, DE 2,421,503, DE 2,526,193, DE 3,142,627, DE 3,108,990, and DE 3,215,591). This method, however, cannot be carried out with diurethanes based on methanol without serious disadvantages since their high melting point is above the decomposition point, and the low solubility necessitates high degrees of dilution. This results in impairment of the volume/time yield, and a considerable increase in costs in the distillative recovery of the solvent.
Aromatic diurethanes and/or polyurethanes are isolated, e.g. according to the teaching of DE 2,917,568 and EP 18,583, from the crude mixtures obtained from the reaction of aromatic diamines and/or polyamines with carbamates. In this process, any catalysts used are removed and any solid products obtained are filtered off. The alcohol and/or the solvent and the carbamate which is optionally used in excess, are completely or partly distilled off, and the product is obtained by crystallization, precipitation, or recrystallization from other solvents. Thus according to EP 18,583 (Example 36) 2,4-bis-(ethoxycarbonylamino)-toluene is purified by distilling off the alcohol used as solvent and excess ethyl carbamate at a reduced pressure of 10 mbar, and dissolving the residue in methylene chloride, and washing it several times with water. Methylene chloride is then separated off, ethanol is added and the mixture is cooled in a mixture of ice and salt. 2,4-Bis-(ethoxycarbonylamino)-toluene then crystallizes with a melting point of 108.degree. to 110.degree. C. However, the 2,4-bis-(ethoxycarbonylamino)-toluene is unsuitable for the decomposition of urethanes due to its insufficient purity. Analytically pure 2,4-bis-(ethoxycarbonylamino)-toluene has a melting point of 134.degree. C.
It was therefore an object of the present invention to provide a process for the preparation of highly pure aromatic diurethanes and/or polyurethanes suitable for the preparation of aromatic diisocyanates and/or polyisocyanates by the thermal decomposition of urethanes.
It was found that in order to obtain highly pure aromatic diurethanes and/or polyurethanes, the reaction known in the art of aromatic diamines and/or polyamines with unsubstituted carbamates with the liberation of ammonia should be carried out with an excess of carbamates, any solvent used being separated off after completion of the reaction, and the product mixture obtained as residue being extracted with water.